This invention relates to a differential detection system and more particularly to a differential detection system with non-redundant error correction for a modulated carrier transmission system conveying multilevel digital signals such as a multilevel differential phase shift keying (DPSK) system and a multilevel continuous phase frequency shift keying (CPFSK) system.
Demodulation systems for modulated carrier transmission system utilizing multilevel digital signals generally include a coherent demodulation system and a differential detection system. The coherent detection system is advantageous in that it can efficiently utilize the received power and has an excellent error rate performance. However, when this system is applied to a TDMA satellite communication system several symbols are necessary for recovering the reference carrier in each of the burst signals thereby degrading the transmission efficiency. On the other hand, with the differential detection system it is possible to decrease the number of symbols for carrier recovery to one so that it is possible not only to improve the transmission efficiency but also to simplify the construction of the apparatus. However, the error rate performance of the differential detection system is poor.
The invention contemplates improvement of the error rate performance of differential detector system while preserving the advantages thereof. To have better understanding of this invention a typical differential detection system will firstly be described briefly. In the prior art differential detection system, a phase comparator is used for comparing the phase of the signal produced by delaying the received signal by one signaling interval with the phase of the signal received in the present signaling interval and a symbol detected by the phase comparator is utilized as a received data symbol. However, with this system the inherent redundancy that a symbol detected by a phase comparator which compares the phase of the received signal delayed by two signaling intervals with the phase of the signal received in the present signaling interval corresponds to a modulo m addition of the data symbol detected in the present signaling interval and the data symbol detected in the preceding signaling interval is not utilized so that the error rate performance of the demodulated output is poor. Even when the inherent redundancy is utilized, the circuit configuration becomes complicated with the result that it has been difficult to operate at high symbol transmission rate in spite of the improvement in the error rate performance.
It is thought that the system disclosed in U.S. Pat. No. 3,529,290 is closest to this invention. The system disclosed in this patent relates to an error detection and correction technique in which additional redundancy essential for the well known parity principle is not utilized. To this end, a signal produced by delaying by one signaling interval the received differentially encoded phase modulated signal and a signal produced by delaying by two signaling intervals the received signal are prepared and these signals are then combined with the signal received in the present signaling interval to form a redundant information necessary for detecting error and correction thereof. In this system, since a circuit for detecting the presence of an error and a circuit for correcting the error are independent from each other, the signal processing system is relatively complicated. Furthermore, this system is constructed to correct the error only when two errors occur consecutively regardless of the type of the errors thus not making use of the informations regarding the type of the errors. As above described, since the signal processing system is complicated it is difficult to increase the transmission speed of the data or symbols.